The infrared spectra of the aryl boronate esters derived from catechol and 2 : 3 dihydroxynaphthalene

SpJpectrochimics Acta,

1964, Vol.

20. pp. 79 to 96.

Pergamon

Prean Ltd.

Printed

in Northern

Imland

The infrared spectra of the aryl boronate esters derived from catechol and 2 : 3 dihydroxynaphthalene F. K. BUTCHER,W. GERRARD,M. HOWARTH,E. F. MOONEY and H. A. WILLIS The Northern Polytechnic, Hollow8y Road, London N.7 (Received

13 Muy 1963)

Ab&r&,-The infrared spectra of 8 series of 8ryl boronate esters, derived from cetechol and 2:3 dihydroxynaphthrtlene(i.e. 2-esyl-benzo-1:3-dioxe-2.boroles (I) end 2-8ryl-naphth~2:3:d& 1:3-dioxa-2-boroles (II) respectively) have been recorded. The principal 8bsorptiOnbands of these compounds in the range 400-1600 cm-l h8ve been resigned. Compsrison with the speotm of the corresponding phosphorus and nitrogen compounds (III and IV) bee permitted the unambiguous rtsaignmentof the B-O, P-O and B-N vibrations. On the basis of the assignmentsmade, 8 new structure is suggested for the boronete esters.

HETEROCYCLIC systems containing boron, together with oxygen, nitrogen and sulphur, have been extensively studied [l], but attention has principally been directed to the study of the U.V. spectra of those systems which are isoconjugate with carbo-cyclic compounds [2]. Very little attention has been given to the infrared spectra of these compounds. The only published data on the 2-phenylbenzo-1 : 3-dioxa-2-borole demonstrated that the spectrum was unlike those of the corresponding 2alkoxy compounds [3]; it was therefore concluded that the electrophilic character of boron was overcome by mesomerism from the 2-phenyl group. In the present work the spectra of 2-aryl-benzo-l : 3-dioxa-2-boroles (I) and of 2-arylnaphtho[2 : 3-4-l : 3-dioxa-2-boroles (II) have been determined. In order to afford unambiguous assignment of the B-O and C-O modes the spectra of the

a / \

’

4 ../-A, ..

I

II

c ‘,I

a

a:’

Vg-ph if’

#\ b/p-ph ..

it

m

lx!

111P. J. MAITLAS,cherrb. Rev. 62, 223 (1962). [2] M. J. 8. DEWAR,V. P. KUBBA and R. PETTIT,J. Chm. Sot. 3076 (1968). [3] J. A. &AU, W. GERRARD,M. F. LAPPERT,B. A. MOUNT~IELDand H. PYSZO~A,J. C&m. Sot. 380 (1960).

19

80

F. K. BUT-,

E. F. MOONEY and H. A. WILLIS 1. C~c~aa~p,M. HOWARTE,

2-phenyl-benzo-l : 3-diazo-2-borole (III) and of 2-phenyl-benzo-l : 3-dioxa-2-phosphorole (IV) have also been recorded. This has also allowed the assignment of B-N, C-N and P-O modes in these oompounds. EXPERIMENTAL The compounds were prepared by interaction of the arylboron dichloride and catechol or 2 : 3 dihydroxynaphthalene [a]. The diaza compound was prepared as previously ‘described [2] by the interaction of phenylboron dichloride and ophenylene diamine (Found: B, 5.6; Calc. for C,,H,,BN,: B, 5.6 per cent). The phosphorole (IV) was prepared by heating catechol (11.53 g, 1.0 mol.) and dmhlorophenylphosphine (18.77 g, 1-Omol) in benzene (100’ml) under reflux for 8 hr, when evolution of hydrogen chloride ceased. Removal of the solvent (~40’/15 mm) afforded a white residue (21.10 g) which on two recrystallizations from benzene afforded pure 2-phenyl-benzo-l : 3-dioxa-phosphorole as colourless needle shaped crystals (Found: C, 66.7; H, 4.2; P, 14.1. C,,H,O,P requires: C, 66.5; H, 4.3; P, 14.3 per cent; m.p. l.68-169°.) The spectra were recorded: in compensated carbon tetrachloride and carbon disulphide solutions (saturated ;solutions were used for the naphthalene compounds (II) due to the low solubility). The spectra were also recorded as Nujol and hexachlorobutadiene mulls, but unless stated the band frequencies quoted are those recorded in solution. A Grubb-Parsons GS2A and Perkin-Elmer (NaCl and KBr) spectrometers were used; the calibration was checked using primary standards [5]. B-O

AND C-O

VIBRATIONSOF THE BOROLES

The B-O and C-O vibrations of phenylboronate esters have previously been considered [6], and although in the present case the two carbon atoms of the o-c -B’ ‘o-c group are joined, it would be anticipated that similar vibrational modes would OCCUF. The four fundamental stretching modes are shown in (V-VIII), (V) and (VI) being the asymmetric and symmetric B-O stretching vibrations. In the

P

XI

m

mI

o-phenylene esters (I) and in the 2( l-naphthyl)-naphtho[2 : 3 : d]-1: 3-dioxaborole (II, Ar = 1-naphthyl) the l”BO and llBO asymmetric stretohing modes are found at 1339 f 6 and 1326 f- 6 cm-l respectively, but in the other 2: 3-naphthalene [4] w. GE-, M. HOWAXTH,E. F. MOONEY and D. E. PRATT,J. Chm. Sot. 1682 (1963). [b] Tablee of wave ilhmbere for the Calibration of Infrared Spectrometem,Butterworths, London (1961).

The infrmed spectra of the aryl boronate e&m

81

esters (II) the bands each occur about 20 cm-l higher in frequenoy. As, in the phenylboronates previously described [0] the weaker higher frequency l”BO modes are only observed as shoulders and in a number of cases the l”BO mode could not be clearly differentiated. The B-O asymmetric mode is easily recognized since it is the strongest band in the spectra. This very high intensity must result from the contribution of the structure I&f) due to the p,,q,, bonding, since in the corresponding 2-alkyl compounds [7], in which this p,,~, bonding does not occur, the B-O asymmetric stretching frequency is a weak band, and difficult to identify. With the exception of the 2-( 1-naphthyl)boroles the BO symmetric modes occur at I.069 f 6 (‘OBO) and 1064 f 6 (IiBO) cm-l. The corresponding modes in (I and II, Ar = 1-naphthyl) occurs at higher frequency ~70 cm-i (see Table 6). Another prominent band in the series of compounds (I and II) is the C-O asymmetric stretching frequency, which remains substantially constant in position in both series of compounds at 1238 f 4 cm-l, a frequency very similar to that in the parent. hydroxy compounds. The symmetric C-O frequency, however is more variable, and largely depends upon the nature of the 2-aryl substituent. (see Tables). The B-O ring deformation modes are easily identified, except for the nz-tolyl esters where there is no evidence of this mode. The ‘OBO and “BO modes remain remarkably constant at 672 f 7 and 658 f 6 cm-l, although the B-naphthyl esters differ from the rest of the series with 700 f 2 and 684 cm-l for the l”BO and llBO modes respectively. For the majority of these esters the BO deformation modes certainly occur at higher frequency than in the non-cyclic esters [6], but at lower frequency than found in six-membered ring cyclic boroxoles [ 81. It is clear that in the 2-aryl-benzo-l : 3-dioxa-2-boroles the BO and CO asymmetric stretching ‘frequencies are relatively constant while the BO symmetric stretch and ring deformation and the CO symmetric stretch are dependent on the nature of the 2-aryl group. THE 2-PHENYL DIAZOBOROLE AND PHOSPHOROLE COMPOUNDS Although comparison of the spectra of this series of similar compounds enables firm assignments to be made for the heterocyclic ring vibrations, excellent confirmation is forthcoming from a comparison with the spectra of other compounds which are similar except that the heteroatoms are separately replaced (Fig. 1). Thus 2-phenyl-benzo-l : 3-dioxa-2-borole (I) may be compared with 2-phenyl-benzo1: 3-diaza-2-borole (III) (oxygen replaced by NH) and with 2-phenyl-benzo-l : 3dioxa-2-phosphorole (IV) in which boron is replaced by phosphorus. Considering tist the bands of (I) assigned as C-O stretching frequencies. These are clearly still present in (IV) but absent in (III), while the two B-O modes, which are very prominent in the spectra of (I), do not appear in either (III) or (IV). [S] J. E. BURCH, W. GERRARD,M. GOLDSTEIN,E. F. MOONEY and H. A. WILLIS, S~ectrcc~m. A&f+, 18, 1403(1962); [?I F. K. BUTCHER,W. GERRARD,E. F. MOONEY,R. G. REES, R. SIKSTROM and H. A. WILLIS. Unpublished work (1963). [S] D. W. AUBREY, M. F. LAPPERTand H. PYSZORA,J. Chem. Sot. 1931(1961).

82

F. K.

Bm,

W.

GEILBABD.

M. HOWARTE, E. F.

MOONEY

and H. A. WILJJS

(b)

1400

1300

1200

1100

1000

900

Fig. 1. Infrared spectra of 2-phenylbenzo-l : 3 diaza-2-borole; 1: 3-dioxa-2-phosphorole and 1: 3 dioxa-2-borole. (a) (c) (e) (g) (i) (k)

and (b) asymmetric and (d) asymmetric and (f) asymmetric and (h) asymmetric and (j) asymmetric B-phenyl stretch.

and and and and and

symmetric symmetric symmetric symmetric symmetric

B-N C-N C-O P-O B-O

stretch. stretch. stretch. stretch. stretch.

The infrared

spectra of the aryl boron&e e&em

83

Conversely, this enables useful assignments to be made for various bands in the spectra of (III) and (IV). The very prominent band pairs near 1430 cm-l and 1350 cm-l in (III) must clearly be the BN asymmetric and symmetric modes respectively, since each shows the correct doublet character and has no counterpart in (I). It is reasonable to suppose that the prominent 1267 cm-l band in (III), again with no counterpart in (I), must be the C-N asymmetric stretching frequency. The corresponding C-N symmetric mode is presumed to be at 1016 cm-l, since this is the only other band of reasonable intensity in this region. In the compounds of the type (IX) previously examined by us [6], the C-N frequencies H I N-C -B

/ \

IX N-C

were doubled, and we considered this to arise from the rotational isomerism possible in these compounds; it is therefore interesting to note that in the present series, in which rotational isomerism is not possible, only single bands are found for each of these vibrational modes. A further comparison of the spectra of (I), (III) and (IV) shows a strong band at 1370 cm-l in (I), with apparently no counterpart in either (III) or (IV). It seems likely that this is the B-phenyl stretch, presumably more prominent in (I) than in (III), since the electron deficiency of boron is more readily overcome by p,,~, bonding from nitrogen than from oxygen, thus giving a B-phenyl bond of higher order in (I) and consequently a more intense band in (I) than in (III); the same mode in (III) is possible the 1309 cm-l band. In the spectrum of the phosphorus compound the C-O frequencies at 1241 and 1030 cm-i have been mentioned. Comparison of this spectrum with (I) and (III) shows clearly that the 866 and 847 cm-i bands must be the P-O frequencies. This is contrary to earlier assignments [9, lo], but agrees well with recent work on trivalent phosphorus esters [ 111. AROMATICRING VIBRATIONS Due to the presence of two different monocyclic aromatic nuclei in these compounds the unambiguous assignment of YC-C and @-H modes is difficult, but where possible assignments have been made (Tables l-5). The problem of the naphthyl compounds is even more difficult since the same detailed assignments have not been laid down as for monocyclic benzene derivatives, however the prominent bands in the 900-400 cm-l region arising from ring vibrations have been considered. [9] R. A. NYQUIST, AppZ. Specirosc. 4, 161 (1967). [lo] L. C. ‘Ikto~s end R. A. CIWJTENDEN, Chem. cl:Id 1913(1961). [ll] A. C. CHAPMAN and R. HARPER, Chem. dz Id 986 (1962).

A&Z--C

C-O

A,BCH A,@H or B,#EH

1286 1277 1269 (v.w.) 1237

1183 1147

-

_ ._

109s 1075 (shf 1070 1026

1094 LO?C(sh) 1070

_ -

1137

1269 1240 1227 1134 11rJ9+

1381 1371 1366(sh) 1348 1318

1388

1456 1440 1430 1418

1499 1484

1605

1135

aaym. stretch

Bl”O B110 mym. stretah 1

1372 1337 (sh) 1329 1316

1400

B-phenylstretch

A,vW A#2----C B,vC--C

1498 1485 1471

1436

A#c-c ApW

1660 1639 1606

I,Ar=Ph

-

asym. stretch

-

--

.-

Iv

B&Z-H B,/?CH Sym. C-O

1072 1030

stretah

_

1064 1026(v.w.)

1103 1086

1214 1179

1267

1363 1343 1309

1381

1440 1426

1486

1604

or B-phenyl

-

asym, stretch

_ -...

C-N

stretch

A, W

and 2-phenyldiazabomle

asym. &retch

B&l-H

C-O

ApC-C B,vG-C

1126 1103 1093

1161

1290 1278 1261 1241 1209

1351 1338

1404

1484 1466 1468 1440

1592

ApfLC 1606 A,vC&C 1621

--- -- -

B&= B’@O B1zO sym. stretoh 1 Sym. C-O stretoh

WCH

.fW= Naphthalena grp. vibration

C-O

B-phenyl stmtah B’OO B110 wym. stretch I

II, Ar = Ph

-

Table 1. Vibrational frequenciesof Z-phenyldioxaboroles,Z-phenyl~oxaph~pho~le

482 448

614*

698 676 659

763 757 749*

819 V.W.

947. 926 (v.w.) 910 (v.w.)* 901 (ah) 865.

WCC

yCH(ring /

rC-H(ring

* Vibration modea of 2 : 3 disubstituted naphthalen ring.

454

696 676 661

742

866 840 820 803 762 V.W.

918

1003

B)

A)

Table 1.

-

556 540

655

690

741

795 766 751

866 a47

1009 984 917

(contd.)

stretch

B&-C

B&-H

B,yC!-H(pheny1)

P-O

B&H

-

663 637 577 564

736 705 696

755 (w)

848

905

1015

86

F. K.

BUTCHER, W.

GERBARD, M. HOWARTH, E. F. MOONEY and H. A. WILLIS

Table 2. Vibrational

frequencies of 2(p-substituted

phenyl)benzo-dioxaboroles

I, Ar = pClC,H,--

I, Ar = pMeC,H,1625(eh) 1615

1618(ah) 1597

A&--C Al&-C ( + B&--C

-

I, Ar = pBrC,H,-

_-

1616 1605 1587 1506(w) 1486 1475(ah)

A,vC-C Al_ B,VC--C

1521

B@--C

1485(sh) 1478(ah)

A&-C

1486(sh) 1475(eh)

A,vC-C

1472

B&---C

1470 1437(w) 1421(w) 1400 1393 1372 1344 1329 1317 1303 1289(w) 1280(w) 1245(w.sh) 1235

BgC-C

1468

B,vC--C

B,+C -c

1420(w) 1399 1389 1364 1345 1332 1318

BpC-4

1401 1382(sh) 1374 1344 1330 1318 1314(sh) 1285(w)

Me asym. and CH, deformation

Sym. CH, deformation ;:;“,

>

asym. stretch

A+-‘2 B,/T?C-H

1247(w.sh) Ssym. C-O 1237 1208(w) 1185 tl48 (w) 1134 1107(w) 1073 1066 1020 1004(w) 920 867 860(sh) 815(sh) 812 775(w) 745 723

stretch

B’OO B”C > stretch C-O eym. stretch B*rCH

A,$‘-H %I/C-H A@-H B&--H +-c

677 661 608

B,,aC-C-C

482

0.0.~. def.

427 421

0.0.~. def.

B’OO B110 asym. stretch

1 ‘4 ,vc-c B,/jC--H

Asym. C-O

1177(w) 1147 1133 1098(w.sh)

d,,9C---H B’ B1l aryl stretch

1091 1070 1065

C---Cl stretch

1015 1004 (w) 918 865 856 (sh) 830 807 745 724 718 671 657 630 575 486 465 461 (sh) 427

4

;;;;} C--C

sym. stretch sym. stretrt1

B&G-H A,$-H

A,yC-H %JC--H C-Cl deformation ,#c-c

A,vC-C

ApC-c

1288(w) B,/.?C -H 1280(w) 1245(m-ah) Asym. C-O 1235 1180(w) 1148 1135 3099 (w) 1089(w) 1074 1056 1013 1005(sh) !)19 866 855(ah) 824 803 145

st,retch

,,!K- H B’ ,~” aryl stratch “1

.4

C -Br stretch U’W svm. stretch C 0 ;ym. stretch &L--H _&yC --H

-H B,yC -H

d,yC-

l#L-- c Cm-Hr duformstiwl #JO B1,O) deformation

0.0.~. def.

721 696 669 656 630 566 479

0.0.~. def.

41’4

0.o.p. dcf

B”‘0 B1lO} deformation B,,aC--C--C

B,,aC-c-c 0.o.p.

dtof.

The i&wed

spectra of the aryl boronate esters

87

Table 3. Vibrational frequenciesof Z(p-substituted phenyl)-naphtho-dioxaborolee II, Ar = pMeC,H,1813

ApCC

1604

MC--C

1620 (ah) 1616 1608 (sh) 1489 (v.w.) 1469 1439 (V.W.) 1418

B&-C ta-c

II, Ar = pClC,H,1608 1697

1692

1612 1607 (ah) 1490 1468 1433 1422 (ah) 1399 (ah) 1388 1376 1360

Bl’O esym. stretch

A,/=-=

1238 1181

AI&J---H

Bl’ my1 &r&ah

1169+ 1140

B” myi stretch

ApC%.C

B@-C

1382 1367 1960

1241 1186 1179 1169* 1139

II, Ar = pBrC,H,-

Asym. G-O

stretch

1610 1604 (ah) 1486 1466 1431

Asym. C-O

&r&oh

1106 (w) 1070 1064 1019 970 V.W. 947 Y.W.’ 911 v.v.w.* 871 (ah) 863* 818 763(w) 746.

C-O

aym. stretch

yCH

(ring A)

B,@---= yCH

(ring B)

723 706 (w) 674 666

w--c

641 618*

B&c-c--c

476

0.0.~. def.

l

B”‘0 B1lO}

deformation

1093 1068 1063 1016 C-O 966 946 w.* 911 v.v.w.+ 863+

y&H

829

%yC=

746* 729 724

+--H C--Cl

gym. stretch

(*g

(riner B) deformation

+-

668 662 616+ 611 477

A)

0.0.~. def,

1387 1370 1366 1349 1267 1269 (eh) 1238 1178 1167 1167. 1161 1138 1097 (xv) 1076 1069

Asym. C-O

stretch

A&-H

C-Br at&ah B”0 eym. stretch

1011 co sym. Stretch 964 946 xv.* 911 v.v.w.+ 881 V.W. 863* aC-H (ringA) 842 (ah) 824 4.$-H 746*

$--=

721 696 666 660

c

deformation

ii::]

deformation

620 616’ 681 476

(rine; B)

Blpc--c--c 0.0.~. def.

Vibrational modea of 2 : 3 dimbatituted naphthalene. BENZO-BOROLES

o-Substituted benzenes normally exhibit one strong band, the B,yC-H mode, at 751 f 7 cm-r [12]. In all the o-phenylene esters (I) studied this mode is found to occur at 744 &- 2 cm-l but in all these esters this band is accompanied by a higher-frequency band (803-815 cm-l) (Fig. 2). T/his latter band, which in the parent o-dihydroxybenzene occurs at 770 cm-l is probably the A,$-H mode (the significance of the position of this band in o-substituted benzenes is discussed [12] R. R. RANDLEand D. H. WHIFFEN,Mol. Spectroec. 111 (1955). This is a publicationof the Institute of Petroleum.

88

F. K. BUTCHER, W. GERRA.RD, ~I. HOW,,~Rm'H, E. F. MOONEY a n d H. A. WILLI8

8

Q0

0,-4

o

)

The infrared specks of the my1 boron&e esters

89

Table 4. Vibrational frequenciesof 2-o-tolyl-benzo- and naphtho[2:3:d]1;3 I, Ar = o-tolyl 1620 1604

1487 (ah) 1481 1472 1452 1440 1422 1397 (sh) 1387 1367 1359 (ah) 1340 (sh) 1325 1318 1284

-

A,&-C

1602 1667 1611 1490

d,vc-C B,y(l--C B&-C

Svm. Me deformation B’O-

B”-0 “) A,vc-c

asym. stretch

1270 (v.w.) 1238 1230 (sh)

c---O asym. stretch

1160 (w) 1148 (w) 1133 1118

B’ R”

1063 1068 1037

l3“J-0 sym. stretch B”--0 > C-0 sym. stretch

4

Aryl stretch

II, Ar = o-tolyl

-4,VC-C

VG-C A,vC-C

1472 1463 1436 1426

B1YC-C

1373 1366 (ah) 1367 (ah) 1346 1297 (w) 1289 (ah) 1284

Sym. Me deformation

1239 1224 1188 1180 1176 (ah) 1167’ 1138

C-O

*,VC--C

aaym. stretch

aaym. stretoh

1117 1072 1058 1037 1019 .(w)

1003 918 868 837 (v.v.w.) 831 (w) 816

d,yC-H

937’ 912” 866*

yC-H(ring

A)

828 _4,yC-H 789

773 763 744 729 679 664 666 (sh) 614 487

B-tolyl B&Z---H WC-H B-tolyl +c-c FP-C B”--0 1 deformation

443 * Vibrational modes of 2 : 3 dieubstituted naphthalane.

763 764 (ah) 749* 729 677 661 654 614* 491 480 437

B-tolyl B,+--H +-H(*g B) B-tolyl #S--C P-0 deform&m ES”--0 )

F. K.

90

BUTCH=,

W.

Table 6. Vibretional

GERRARD, M. HOWARTH, E. F. MOONEY and H. A. Wmms

frequencies of Z-m-tolyl-bmzo

and naphtho[2:3:d]l:3

I. Ar = m-tdyl 1839 1623(ah) 1816(sh) 1610 1686 1487 1478(w) 1473

A,vG-C

1433(ah) 1427(ah) 1418 1410(sh) 1394(w) 1387(w) 1384(w) 1308 1348(w) 1333(ah) 1323 1317(w) 1236

B,vC-C

1468 1461(eh) 1432

Bym. Me deform

BIP-H C-O asym. stretch

B’ Bll

4

aryl stretch

Bib0 B110 eym. stretch

917 900 874 862 861 833 816 791 773 744 711 (ah) 706

A*_ B&--C A,VC--C

B,vC--C

1419 1403

1206

A&--= WC--= --UC--=

667 660V.W. 629V.W. 490V.W. 432 l

II, Ar = m-to1y1

1612 1606 1680 1610 1487

1148 1130 1096 1086 1072 1068 1004

dioxeboroles

Vibrational modeaof 2:3 disubstitutid naphthalene.

1380 1371 1367(ah) 1360 1340(ah)

Sym. b& deform

CO

1239 1226(sh) 1204 1190 1182 11582 1138

B” aryl stretch

1070

B”0 sym. stretch

1018 947. 920 9112 898 (V.W.) 889 866* 843 830

C-O

792 762(sh) 746* 710 (sh) 706 677 664 614’ 626 499 479

asym. stretch

sym. stretch

-%yC--R J%yC---H @--H(ring

A)

4P--H yC-H(ring

fW--C

B)

The in&red spectra of the aryl boronate esters

elsewhere [13], where it is demonstrated that this band arises when thereis electronic interaction between two ortho-substituents).

91 strong

2 : 3 NAPHTHALENE ESTERS The characteristic absorption frequencies? of substituted naphthalenes have been considered [14], and in this work it was clearly demonstrated that each ring could be treated independently. Consequently in 2 : 3 disubstituted naphthalene derivatives a pattern of bands arising from four hydrogens in positions 1,2,3 and 4 (i.e. equivalent to o-substituted benzene) and from two isolated hydrogens in the 1,4 positions (i.e. equivalent to 1: 2: 4: 5-tetra substituted benzene) is expected.

4

4

0

In all the 2-aryl-naphtho[2 : 3 : d]-1: 3-dioxa-2-boroles examined in the present work there are six bands which appear to be characteristic of this ring structure, these bands occur at 1158 f 1, 948 f 2, 911 f 1, 865 f 2, 748 f 2 and 616 &2 cm-l. Of these bands only two correspond in any way to the nine previously suggested as characteristic of 2: 3 disubstituted naphthalenes. The first band at 1158 f 1 cm is just outside the frequency range given for the 1: 2: 3: 4 hydrogen pattern 1110-l 156 cm-l and also for the 1:4 hydrogen pattern 1126-l 157 cm-1 [14]. The 865 & 2 cm-l corresponds very closely to the frequency given for the yC-H mode of 1: 2 : 4 : 5 tetrasubstituted benzenes [12], and consequently this band is assigned to the out-of-plane C-H deformation of ring A. This also presumably corresponds to the 870-889 cm-l band found in the 1: 4 hydrogen pattern of naphthalenes [L4]. The 748 + 2 cm-’ band also corresponds to the C-H out-of-plane deformation of ‘four adjacent hydrogen atoms of ring B, which in o-substituted aromatics occurs in the range 751 & 7 cm-l [12]; this again probably corresponds to the 726-770 cm-l band reported for the 1: .,: 9 3 : 4 hydrogen pattern in naphthalenes [ 141. The 616 f 2 cm-l is probably a ring deformational mode, and the weak !M8 i 2 and !)11 I) 1 cm-l bands are probably inplane C-H deformational modes. VIHRATI~NS OF THE

B-ARYL

GROUP

(a) Phenyl The B&--K mode (751 f 15 cm-’ [12]) is coincident with the B&H mode of the o-phenylene ring and the yClH mode of the naphthalene ring; the B&C-C mode at 697 f 1 cm-’ is in the normal frequency range (697 f 11 cm-l [12]). In the phosphorole and the diaza compounds the B.&J--N mode of the phenyl nucleus can be differentiated from the B&Z-H mode of the o-phenylene ring as in these two compounds this latter mode occurs at lower frequency (Table 1). (b) p-Substituted phenyl The strong B,,,yC---H mode is found in the 812-830 cm-l range (Tables 2 and 3), which is in the normal range of p-substituted aromatics (817 & 13 cm-l [12]). [13] E. F. MOONEY. Unpublished work, Spectrochim. A&u, submitted for publication. 1141 J.G. HAWKINS.FJ.R. WARD and D.H. WHIFFEN,&I~~~T~&~TPL Actu,lO, 106 (1967).

92

F. K. BUTCHER, W. GERRARD, M. HOWARD,

The #C-C mode is found at 720 f 3 cm-i, 482 f 4 cm-l (I) and at 476 rfr:1 cm-l (II).

E. F. MOONEY and H. A. WILLIS

and the out-of-plane

deformation

at

(0) o- Tolyl The B-o-tolyl group appears to have the usual characteristic frequencies for the B&!-H mode (at 763 cm-f), and for the As&-C mode (at 729 cm-l). Both of these frequencies are similar for the corresponding modes in o-tolylboron dichloride, i.e. 758 and 720 cm-l [15]. In these compounds two distinct o&osubstitution patterns are obtained, one in the normal range from the B-o-tolyl group and the other at lower frequency arising from the o-phenylene nucleus in (I) and the 2: 3 naphthalene nucleus in (II) (see Figs. 2 and 3). (d) m- Tolyl The B,yC-H and the B&!-C modes occur at 791 f 1 and 705 f 1 cm-1 respectively, which, like the corresponding modes in m-tolylboron dichloride, are in the normal frequency ranges of m-substituted hydrocarbons (782 f 9 and 690 f 15 cm-l [12]).

(e) 1-Naphthyl As with the 2 : 3 disubstituted naphthalene derivatives vibrations are found to correspond to the two differently substituted rings, ring A’ (corresponding to a 1: 2 : 3 trisubstituted benzene) and ring B’ (corresponding to a 1: 2 disubstituted benzene). The yC-H mode of ring A’ occurs at 785 & 3 cm-l (Table 6) and is in close agreement with the same mode of 1: 2 : 3 tri-substituted benzenes [12], and probably corresponds to the 795 cm-i band of the 1: 2: 3 hydrogen pattern of napht’tielenes [14]. In addition both the 2-(1-naphthyl) compounds show prominent

Both of these are probably bands at 510 f 1 and 478 f 2 cm-l. deformation modes; the bands are clearly shown in Figs. 2 and 3. THE

STRUCTURE

OF .%ARYL-BENZO-1:

out of plane

%DIOXA-%BOROLES

Previous investigation [3] of the spectrum of the 2-phenyl compound (I) has led to the suggestion, based upon the difference between the spectra of the 2-elkoxy and the 2-phenyl compounds, that (X) was the major contributing structure in compensating the electron deficiency of boron. However it is now suggested that there is very little contribution from structure (X) and that (XI) and (XII) must be regarded as the principle canonical structures. This hypothesis is based upon the following facts. [16] F. K. BUTCHER, W. GERRARD, M. HOWARTE, E. F. MOONEY and H. A. WILLIS. Spwtrochim. Acta. 19, 905 (1963).

The infrared spectra of t h e ary! boronate esters

98

o/

m

b

~

d

a

L

(hi

(e} o\

(d)

d b d

C ~ 4 . -

. I . ~ . ~ L

(e}

(f}

c

I

I

m

n ~>-Q

I

Fig. 3. Infrared specgra of 2-aryl-naphtho[2 : 3 : d]- 1 : 3-dioxa-2-boroles. (a) r C - - H mode ring A. {p. 91). (d) B-o-Tolyl ring modes. (b) r C - - H mode ring B. (p. 91). (e) 1-Naphthyl ring modes. (c) B-O-ring deformation.

94

F. K. Bxmcnimt,W. UEBEABD. M. EOWARTE, E. F. MOONEY and H. A.

Table 0.

vibretionel

frequencies of Z-( I-naphthyl)bemo-

8nd naphtha2:3:d]l:3

ApC-C 1676 1613 1607 1496

1480 1472 1484 1416 1406 1370 1361 1344 1333 (ah) 1322 1318 1287 1279 1286 1267 1236 1212 (w) 1204

1366 1360 1336 (sh) 1327 Al-

Asym. C-O

stretch

PC!-H(ring PC-H(ring

A’) B’)

Sym. C-O

&r&ah

917 904 870 860 836 818 806 788

1279 1269 1266 1240 1226 (w) 1202 1180 1167. 1139 1131

Asym. G-O

&e&oh

1068 1019 977 964 (w) 9492 9112 869’ 866 (w) 836

$-Wing

4

c@--C(ring @-C(ring

A’ and B’) A and B)

806 782 769 (ah) 746. 739 699 682

746 702 686 670 660 617 682 646 611 476>

VG-C

1466 1446 1418 1403

1140 (w) 1130 1083 (w) 1068 1024 1004 977 964 (w)

dioxeborole

II. Ar = I--C,,H,-

I, Ar = l-C,,H,1621 1698 1676 1610

WILLIS

+C--C(ring

A’ and B’)

648 616’ 637

I-naphthyl ring modes

428 IDVibrational modes of 2:3 disubstitutad naphthalene.

::a 466

l-naphthyl ring modus

The infrared spectm of the ayl boron&a eatam

90

(a) The yC-H modes of the B-aryl substituent groups are substantially in the normal ranges as for the corresponding substituted aromatio hydrooarbona [12] and hence there can be but little mesomeric interaction (X) between the aryl group and the boron atom in overcoming the electron defioienoy of the latter. Sinoe in compounds, where mesomerism from the aromatio nucleus overcomes the electrophilic bharacter of boron, the J&-H modes are severely modified. For example in B-triphenylborazole (C,H,BNH), the yC-H and &!-C modes ooincide at 702 cm-l, which is identical to the frequency found in monosubstituted aromatio compounds which have a deactivating substituent group [16]. Similarly in B-tri-p-tolyl borazole (IPCH,C,H,BNH), the yC-H mode occura at 736 cm-l, which is at a lower frequency than the same mode in Ip-substituted aromatio hydrocarbons, but is comparable to the same mode in p-halogenonitrobenzenes at 742 f 3 cm-l [13]. (b) The high intensity of the B-O asymmetrio stretoh has aheady been explained in terms of B-O p,,-p, bonding, and this B-O bonding ie reinforced by c)-C=C--t) mesomerism in the boronates (XI and XII). Thie latter effeot ie confirmed by the presence of the 803-816 cm-l band, whioh arises when there ie strong electronic interaction between two optho-groups (see text).

x

(c) The canonical structures (XI and XII) are also consistent with the C-O asymmetric frequency and the high intensity of this band. [16] W. GERRARD,E. F. MOONEY and H. A. WILLIS, J. Ckm.

Sot. 3163 (1961).